Gonsalves: A creepy crawly mystery

Monday

Oct 28, 2013 at 5:29 PMOct 28, 2013 at 6:50 PM

Mounds of lifeless bodies lined Kalmus Beach in Hyannis. There had to be tens of thousands of them.It was Hyannis resident Beverly Mayo who first alerted me by email Sunday afternoon, complete with photos of the creepy sight. The next morning I had to go see for myself.

Sean Gonsalves

Mounds of lifeless bodies lined Kalmus Beach in Hyannis. There had to be tens of thousands of them.It was Hyannis resident Beverly Mayo who first alerted me by email Sunday afternoon, complete with photos of the creepy sight. The next morning I had to go see for myself.

The area wasn’t cordoned off with yellow tape, so I was able to walk up and get a good, close look. Stooping over the alien-looking carcasses, I had to fight off the heebie-jeebies. Never seen anything like it – a foot-wide swath of once creeping, crawling spider crabs strewn across the length of the beach like an ominous tide line.

Who, or what, dunnit?

Suspect No. 1: Toxic chemicals in the oceans? Just think of all that fertilizer and pesticide used to keep those big lawns along the coastline looking so lush. And what about all this talk of wastewater pollution?

Suspect No. 2: Global warming? We all know the waters in and around Cape Cod Bay are warmer. It’s affected the plankton that cod eat – possibly one of the primary reasons the fish that put us on the map can’t be found around these parts any longer, fishery scientists suspect. The waters are warm enough to force the owners of the Pilgrim Nuclear Power Station in Plymouth to power down the plant for several days this summer because the seawater they use to cool the reactor exceeded the temperature the plant license stipulates.

Speaking of which ...

Suspect No. 3: Maybe the Pilgrim nuke plant leaked out some radiation into Cape Cod Bay that made the crabs sick and caused a massive die off?

As scary and perhaps even plausible as those three “suspects” may appear, it turns out none of them are responsible for the mass crab death on Kalmus Beach.

Why? Because there was NO mass casualty of spider crabs. The thousands upon thousands of bodies – or rather body shells – at Kalmus don’t belong to dead crabs. The crabs are alive and well, somewhere.

The explanation behind what looks like a crabatorium of biblical proportions? It’s molting season.I sent photos and an on-scene description to David Remsen, manager of the Marine Resources Department at the Marine Biological Laboratory in Woods Hole.

“I believe those to be moults (or molts) of the spider crab, Libinia emarginata. This species appears to have a very tightly tuned molting cycle where they appear to do it all at once. We had a similar, albeit smaller, event in our own tanks last year,” he wrote to me in an email.

Then he gave me a fascinating bit of insight into how this “coordinated molting” is a dangerous time for crabs and what it may mean in evolutionary terms.

“Their soft shell makes them more vulnerable to predation than with a hard shell. They also can be very slow and unresponsive after a molt, increasing their vulnerability,” he wrote.

“A simultaneous molt,” he explained, may have at least two evolutionary explanations, one more likely than the other.

“When I saw it occur in our tanks I wondered if the molting crabs might release some chemical that induces molting in nearby crabs. If everyone around is going through the molt at the same time, then the chance of a neighbor eating you while you are down is reduced. I’m less confident in this possibility because evolution doesn’t generally work for the common good. It favors the survival of individuals.”

A second possibility, he explained, is that by molting all at once, the Libinia population exposes their vulnerability to predators all at once.

“This provides a temporary glut of vulnerable individuals that provide a huge but short-lived feast for nearby predators. Similar to a cicada irruption, there are simply too many individuals to eat all at once and so many more survive than are eaten.”

He left me with one last telltale sign, corroborated independently by Liz Lewis with the Barnstable Department of Natural Resources.

Remsen said if what I saw were “truly dead crabs,” they would have reeked like the funk of 40,000 years, with lots of flies buzzing around.

But at Kalmus, there was no foul odor or flies. Even the two or three seagulls out there on the beach with me weren’t interested. And if the seagulls aren’t interested, there’s nothing there to eat.

Natural Resources assistant and part-time New England Aquarium educator Liz Lewis went out with a crew of scallop surveyors on Friday just off Kalmus. They dredged up many molted spider crab shells in their nets.

When they started receiving calls from concerned citizens about dead crabs on the beach, she went out to investigate.

“Many of them you could feel the soft shell, which is indicative of molting. Plus, dead ones would really smell,” she said.

In other words, aside from the size of the molt shake at Kalmus, there’s nothing to be alarmed over. “Though I will say, I’ve never seen anything like that (at Kalmus) before,” she said.

You gotta love Cape Cod. Where else are you going to get Mother Nature to present a creepy mystery on a beautiful beach, days before Halloween?